Ring laser semiconductor spectroscopy system

ABSTRACT

An optical power control system for a semiconductor source spectroscopy system controls power fluctuations in the tunable signal from the spectroscopy system and thus improves the noise performance of the system. This general solution has advantages relative to other systems that simply detect reference power levels during the scan and then correct the detected signal after interaction with the sample by reducing the requirements for coordinating the operation of the sample detectors and power or reference detectors. The spectroscopy system comprises a semiconductor source and a tunable filter. The combination of the semiconductor source and tunable signal illuminate a sample with a tunable signal, being tunable over a scan band. The power control system comprises an amplitude detector system for detecting the power of the tunable optical signal and power control system for regulating the amplitude of the tunable optical signal in response to its detected power.

RELATED APPLICATIONS

This application is related to Application Serial Nos. (Attorney docket0005.1130US1 and 0005.1133US1), filed on an even date herewith, byFlanders, et al.

BACKGROUND OF THE INVENTION

Most spectroscopy systems fall into one of two categories. They can betunable source systems that generate a tunable optical signal that isscanned over a scan band. A detector is then used to detect the tunableoptical signal after interaction with the sample. The time response ofthe detector corresponds to the spectral response of the sample. Suchsystems are typically referred to as pre-dispersive. Alternatively, atunable detector system can be used. In this case, a broadband signal isused to illuminate the sample. Then, a bandpass filter is tuned over thescan band such that a detector time response is used to resolve thesample's spectrum. Such systems are typically referred to aspost-dispersive.

Among tunable source and tunable detector systems, tunable sourcesystems have some advantages. They can have a better response for thesame optical power transmitted to the sample. That is, tunable detectorsystems must illuminate the sample with a broadband power signal thatcovers the entire scan band. Sometimes, this can result in excessivesample heating. Also high power is generated at the optical source, mostof it being used, making the system inefficient. In contrast, at anygiven instant, tunable source systems only generate and illuminate thesample with a very narrow band power within the scan band.

Further, tunable source systems have advantages associated withdetection efficiency. Relatively large detectors can be used to capturea larger fraction of the light that may have been scattered by thesample, since there is no need to capture light and then collimate thelight for transmission through a tunable filter or to a grating and adetector array.

A number of general configurations are used for tunable sourcespectroscopy systems. The lasers have advantages in that very intensetunable optical signals can be generated. A different configuration usesthe combination of a broadband source and a tunable passband filter,which generates the narrowband signal that illuminates the sample.

Historically, most tunable lasers were based on solid state or liquiddye gain media. While often powerful, these systems also have high powerconsumptions. Tunable semiconductor laser systems have the advantage ofrelying on small, efficient, and robust semiconductor sources. Oneconfiguration uses semiconductor optical amplifiers (SOAs) andmicroelectromechanical system (MEMS) Fabry-Perot tunable filters, asdescribed in U.S. Pat. No. 6,339,603, by Flanders, et al., which isincorporated herein by this reference in its entirety.

In commercial examples of the broadband source/tunable filter sourceconfiguration, the tunable filter is an acousto-optic tunable filter(AOTF) and the broadband signal is generated by a diode array ortungsten-halogen bulb, for example. More recently, some of the presentinventors have proposed a tunable source that combines edge-emitting,superluminescent light emitting diodes (SLEDs) and MEMS Fabry-Perottunable filters to generate the tunable optical signal. See U.S. patentapplication Ser. No. 10/688,690, filed on Oct. 17, 2003, by Atia, etal., which is incorporated herein by this reference in its entirety. TheMEMS device is highly stable and can handle high optical powers and canfurther be much smaller and more energy-efficient than typically largeand expensive AOTFs. Moreover, the SLEDS can generate very intensebroadband optical signals over large bandwidths, having a much greaterspectral brightness than tungsten-halogen sources, for example.

SUMMARY OF THE INVENTION

In general according to one aspect, the invention features a hybridcavity semiconductor laser comprising: an semiconductor opticalamplifier in a laser cavity, a tunable filter in the laser cavity, and afiber forming part of the laser cavity. The fiber is polarizationcontrolling fiber, such as polarization maintaining fiber or fiber thatpropagates only a single polarization.

The invention also features a tunable ring laser comprising asemiconductor optical amplifier in a ring cavity and a Fabry-Perottunable filter in the ring cavity. Prisms are used to return the lightto recirculate through the ring cavity.

A cavity length modulator is used to control an optical length of theoptical cavity.

The invention also features a laser system comprising a laser cavitycomprising a free-space portion and an optical fiber portion, asemiconductor gain medium in the free-space portion, and a Fabry-Perottunable filter in the free space portion to control a wavelength ofoperation of the laser system to scan over a scan band tospectroscopically analyze a target.

The above and other features of the invention including various noveldetails of construction and combinations of parts, and other advantages,will now be more particularly described with reference to theaccompanying drawings and pointed out in the claims. It will beunderstood that the particular method and device embodying the inventionare shown by way of illustration and not as a limitation of theinvention. The principles and features of this invention may be employedin various and numerous embodiments without departing from the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, reference characters refer to the sameparts throughout the different views. The drawings are not necessarilyto scale; emphasis has instead been placed upon illustrating theprinciples of the invention. Of the drawings:

FIG. 1 is a perspective view of a tunable source semiconductorspectroscopy system with a wavelength and amplitude referencing systemaccording to the present invention;

FIGS. 2A through 2D illustrate a number of examples of the tunablesemiconductor source for use with the present invention;

FIG. 3 is a plot of power (decibel scale) showing the gain spectrum of asemiconductor source as a function of wavelength;

FIG. 4 is a plot of transmission (decibel scale) showing the gainspectrum of the semiconductor source and the passband of the tunablefilter;

FIG. 5A is a plot of signal optical power as a function wavelength for asemiconductor source using the inventive spectral rolloff compensationand noise suppression;

FIG. 5B is a plot of transmission noise as a function wavelength for asemiconductor source without using the inventive noise suppressionsystem;

FIG. 5C is a plot of transmission noise as a function wavelength for asemiconductor source using the inventive noise suppression system;

FIG. 6 is a plot of noise as a function of wavelength showing the noisesuppression gained from current control of the semiconductor chip;

FIG. 7A is a circuit diagram showing an inventive analog drive circuitfor the semiconductor source;

FIG. 7A-1 is a block diagram showing the operation of the inventivedrive circuit;

FIG. 7B is a circuit diagram showing an inventive analog drive circuitfor the optical modulator;

FIG. 8 is a perspective view of a linear cavity tunable laser accordingto the present invention;

FIG. 9 is a perspective view of a linear cavity tunable laser accordingo the present invention;

FIG. 9A illustrates the optical train between the SOA chip and thetilted Fabry Perot tunable filter, according to the invention;

FIG. 10 is a schematic view of a ring cavity tunable laser according othe present invention;

FIG. 11 is a perspective view of the ring cavity tunable laser accordingo the present invention;

FIG. 12 is a perspective view of a multi-chip tunable SLED systemaccording to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a semiconductor source spectroscopy system 100, which hasbeen constructed according to the principles of the present invention.

Generally, the spectroscopy system 100 comprises a tunable semiconductorsource 200. This generates a tunable optical signal 210.

In one example, the tunable signal 210 is transmitted to a frequencyreference tap 310 that diverts a portion of the tunable optical signal210′ to an optical frequency or wavelength reference 312. In oneexample, this optical reference is a fixed cavity etalon that provides anumber of spectral passbands located within and/or spectrally adjacentthe scan band of the system 100. Optionally, a post-amplifier trackingtunable optical filter is sometimes used to filter out or remove anyoptical noise contributed by the amplifier.

The signal 210′ that is transmitted through the optical reference 312 isthen detected by a frequency reference detector 314. The output of thefrequency reference detector 314 is sent to an analog to digitalconverter 450 of a controller 410. This enables the controller 410 todetermine the instantaneous frequency of the tunable signal 210 tothereby provide frequency or wavelength calibration for the spectroscopysystem 100.

In some implementations, the tunable optical signal 210 is thenamplified in an optical amplifier 316. In one example, this opticalamplifier can be a rare-earth doped amplifier, such as a fiber amplifieror waveguide amplifier. In a more preferable embodiment, a semiconductoroptical amplifier is used to boost or increase the power of the tunableoptical signal 210 while not significantly increasing the overall sizeof the system 100.

The tunable optical signal 210 is then transmitted to an optical signalpower regulator 318 of the power control system. This modulates thepower of the tunable optical signal 210.

In one example, the optical signal power regulator comprises an SOA thatselectively amplifies or attenuates the tunable optical signal 210 bycontrol of the SOA drive current. In this example, the optional SOAamplifier 316 would usually not be used.

In the preferred embodiment, the optical signal regulator or modulator318 is a variable optical signal attenuator. Preferably, it is a solidstate, high-speed optical attenuator. One commercial example isavailable from Boston Applied Technologies, Inc. These areelectro-optical, ceramic devices that are electrically modulated inorder to control the level of the optical attenuation applied by theregulator 318 to the tunable signal 210.

The power detector 322 detects the amplitude of the tunable opticalsignal from the optical signal regulator 318. Specifically, a portion210″ of the tunable optical signal 210 is preferably diverted by a powertap 320 to a power detector 322 of the power control system. Thedetected level of the tunable signal 210 is then provided to thecontroller 410, and specifically its drive circuit 411.

The remainder of the tunable optical signal 210 is sent through to thesample 10 and the sample detector 12. In some examples, the output ofthis sample detector 12 is also provided to the controller 410, which isthen able to resolve the spectrum of the sample 10 by resolving the timeresponse of the sample detector 12 into the spectral response of thesample 10.

The drive circuit 411 of the controller 410 is used to regulate thepower to the tunable semiconductor source 200 or the optical signalregulator 318, or both. Specifically, the output of the power detector322 provides a power feedback signal that the drive circuit 411 of thecontroller 410 uses in order to stabilize the level and reduce amplitudenoise of the tunable signal 210 in spite of noise such as RIN noise ormode-hopping noise. The controller drive circuit 411 regulates thetunable semiconductor source 200 and/or optical signal attenuator 318 inorder to stabilize or set the power in the tunable optical signal 210over the scan band.

FIGS. 2A-2D show a number of examples of the tunable semiconductorsources 200 that are used in embodiments of the inventive spectroscopysystem 100.

Specifically, FIG. 2A shows a first linear cavity laser embodiment(200-1) of the tunable semiconductor source 200. This is generallyanalogous to the tunable laser described in incorporated U.S. Pat. No.6,339,603.

Specifically, light is amplified in an SOA 610. This light is filteredby an intracavity Fabry-Perot tunable filter 612. In one embodiment, theFabry-Perot tunable filter is manufactured as described in U.S. Pat.Nos. 6,608,711 or 6,373,632, which are incorporated herein by thisreference.

Out-of-band reflections from the filter 612 are isolated from beingamplified in the SOA 610 by a first isolation element 614 and a secondisolation element 616, on either side of the filter 612 in the opticaltrain. In different implementations, these isolation elements 614, 616are isolators or quarterwave plates. The laser cavity is defined by afirst mirror 618 and a second mirror 620. In some implementations, areflective SOA 610 is used, which provides the reflectivity of the firstmirror 618 at one end of the cavity. The tunable signal 210 is emittedthrough the second mirror 620 in one example.

In some embodiments, a portion of the laser cavity includes a length ofoptical fiber 615. The second mirror 620 is then typically a discretemirror or a fiber Bragg grating reflector that is formed in the fiber615. The advantage of using the hybrid freespace/fiber laser cavity isthat the laser cavity can be made long, typically longer than 10centimeters, and preferably long than 0.5 meters. The long cavityprovides for tight longitudinal mode spacing to reduce mode hoppingnoise.

In a current embodiment, the SOA chip 610 is polarization anisotropic.Thus, polarization control is desired to stabilize its operation. Assuch, fiber 615 is polarization controlling fiber such a polarizationmaintaining or is fiber that only transmits or propagates a singlepolarization.

FIG. 2B shows another implementation (200-2) of the linear cavitytunable laser functioning as the tunable semiconductor source 200. Here,an SOA 610 is used in combination with a first mirror 618 and a secondmirror 620. The SOA 610 is isolated from the out of passband reflectionof the Fabry-Perot tunable filter 612 by tilt isolation. Typically theangle α between the optical axis OAF of the filter 612 and the opticalaxis of the laser cavity OAC is less than 5 degrees, and preferablybetween 1 and 3 degrees. Currently, angle α is about 1.3 degrees. Inthis way, the system generally avoids the out-of-band reflections frombeing amplified in the SOA 610. Preferably the tunable filter 612 hasflat-flat mirror cavity to further improve isolation.

In this embodiment, a hybrid freespace-fiber cavity is used in someimplementations to provide the long optical cavity/tight mode spacingcharacteristics by further including the fiber length 615.

In a current embodiment, the SOA chip 610 is again polarizationanisotropic. Thus, polarization control is desired to stabilize itsoperation. As such, fiber 615 is polarization controlling fiber such apolarization maintaining or is fiber that only transmits or propagates asingle polarization.

FIG. 2C shows another implementation (200-3) of the tunablesemiconductor source. Here, a SLED 622 is used to generate a broadbandsignal that is then filtered by a Fabry-Perot tunable filter 612 inorder to generate the tunable signal 210. In other implementations, anintervening isolator 624 or tilt isolation is used to isolate the SLED622 from the back reflection of the Fabry-Perot tunable filter 612.Further, incorporated U.S. patent application Ser. No. 10/688,690, filedon Oct. 17, 2003, by Atia, et al., describes some other variants, whichare used in still other embodiments, depending on whether increased scanband or power is required.

Finally, FIG. 2D shows another implementation (200-4) of the tunablesemiconductor source 200. This also combines an SOA 610 and aFabry-Perot filter 612. A ring cavity laser, however, is used.Specifically an optical fiber or bent beam path 626 is used torecirculate the light back through the SOA 610 for furtheramplification.

FIG. 3 is a plot of the power (arbitrary units, decibels) for a SLED orSOA as a function of wavelength in nanometers, which applies to thevarious embodiments of the semiconductor tunable source 200 in FIGS.2A-2D. This plot shows how the power output from the system isstabilized across the scan band.

Generally, the unmodulated SLED or SOA power spectrum will peak at somewavelength, here approximately 1680 nanometers (nm). This wavelength isusually dictated by the epitaxial structure of the devices along withthe material system that is used. The power, however, can vary bygreater than −40 dB over the desired scan band 510, extending from about1610 nm to 1770 nm, in this one example. This is due to the fact thatthe semiconductor gain medium does not generate all wavelengths withinthe scan band with equal efficiency. Moreover, the power in a narrowslice of the spectrum varies over time due to thermal noise and RIN.

The power to the tunable semiconductor source 200 and/or the opticalsignal regulator 318 are preferably controlled to stabilize the power ofthe tunable signal 210 transmitted to the sample from the semiconductorsource 200 so that it is stable or at least known across the entire scanband of approximately 1610 to 1770 nm, in one exemplary scan band. Inthis way, the rolloff in the power spectrum is compensated. As a result,where an attenuator optical power regulator is used or the power to theSLED/SOA is modulated, the tunable optical signal power is attenuated orthe power from the semiconductor source is lowered in order to achieve astable or constant tunable signal power. However, the total power fromthe system 100 is lower than it would be otherwise be capable ofgenerating, see area 511. In this way, the present system can be used tostabilize the output power of the source due to spectral roll-offassociated with limitations in the gain band of the semiconductorsources.

FIG. 4 is a plot of several tunable filter passbands 512 as a functionof an exemplary SLED or SOA power spectrum 514 over a scan band 510stretching from approximately 1340 to 1460 nm. This plot shows therelationship between the tunable passband of the Fabry-Perot tunablefilter 612, the semiconductor source gain spectrum 514, and theoperation of the inventive power control system.

As a result, without modulating the power of the tunable optical signal,the output power filling the passband 512 of the tunable filter 612would peak at about 1400 nm. However, by modulating the power to theSLED 622 or SOA 610 or by controlling its attenuation using opticalsignal modulator 318, the output power can be stabilized to the level516. Moreover, by modulating the tunable signal power at high speed,temporal amplitude fluctuations in the tunable signal 210 are mitigatedor removed. This has the effect of increasing the SNR of the system andreducing the noise power over the scan band.

FIG. 5A is a plot of the response of the power detector 322 as afunction of the wavelength across the scan band of semiconductorspectroscopy system 100 showing the performance improvement provided bythe present invention.

These data were taken from the embodiment that used the modulation ofthe optical signal attenuator 318 in order to improve the noiseperformance of the system.

Specifically, plot 413 shows the response at the sample detector 12 withthe attenuator 318 not being driven, i.e., in a transmissive state. Nosample 10 is present for this experiment. The spectrum generally peaksaround 1,700 nm, which is the center of the gain band of thesemiconductor source 610. Further, the power exhibits a high degree ofvariability across the scan band 510.

In contrast, data 412 illustrate the response at the sample detector 322with the attenuator 318 being driven by drive circuit 411 in order toflatten the response. Specifically, the response is generally flatacross an entire scan band from approximately 1620 to 1780 nm, showingonly a small degree of ripple, which is believed to be attributable topolarization sensitivity in one of the taps. Most importantly, the highfrequency variability has been reduced, improving the noise performanceof the system.

Of note is the fact that the power output from the system is generallylower when the attenuator 318 is in operation. This is because itachieves the spectral flatness and stability by selectively attenuatingthe tunable optical signal 210.

FIG. 5B is a plot of normalized transmission response (100% baseline) asa function of wavelength across the scan band with the attenuator notoperating. Data 414 exhibits a relatively noise spectrum.

FIG. 5C shows the spectral response with when the attenuator is beingdriven to stabilize the output. Specifically, data 416 show thetransmission stabilizing around the normalized value of 1 on theprovided scale.

FIG. 6 is a plot of the response of the sample detector 12 as a functionof the wavelength across the scan band of semiconductor spectroscopysystem 100 showing the performance improvement provided by the presentinvention by using current control to the semiconductor SOA chip 610 orSLED chip 622 in order to improve the noise performance of the system.

Specifically, plot 550 shows the normalized transmission response at thesample detector 12 in absorbance units(absorbance=Log₁₀(transmission⁻¹). The spectral noise has beensubstantially reduced. In the illustrated example, the SNR was greaterthan 44000 RMS. However, the noise performance did degrade in regions552 and 554, possibly due to a broadening of the tunable filter'spassband in these spectral regions.

FIG. 7A is a circuit diagram showing the semiconductor source drivecircuit 411. This is used in the embodiments that control the current tothe SLED 622 or SOA 610 in order to achieve a power-stable tunablesignal 210.

The circuit comprises a transimpedance amplifier 670. This element isnot always necessary. In the current embodiment, it is used for commonmode rejection of noise sources, such as 60 hertz interference.

The voltage across the power detector photodiode 322 regulates the inputto an integrating amplifier 672. Specifically, integrating amplifier 672is used to control the loop bandwidth. By changing capacitor C1, thebandwidth response of the circuit can be adjusted.

Sub-circuit 674 is used to control the biasing of the photodiode 322.Capacitor C2 (reference numeral 676) controls the frequency response ofthe circuit.

In the illustrated embodiment, two power transistors 678 and 680 areused to control the power to the anode of the SLED 622 or SOA 610.

FIG. 7A-1 is a block diagram illustrating the operation of the drivecircuit 411. Specifically detector 322 provides the feedback signal todrive circuit 411. The drive circuit signal is then summed with acurrent set point signal 413. This is the power control signal to theSOA or SLED 610, 622.

The feedback path 411-A of the controller 411 functions an integrator.The current implementation, it is stable until the second pole of theoperational amplifier.

FIG. 7B is a circuit diagram showing the drive circuit 411 for themodulator/attenuator 318. This circuit similarly has a trans-impedanceamplifier 670 to assist in common mode rejection. The power detectorphotodiode 322 provides the input to an integrating amplifier 686.Capacitor C4 (688) is used to control the loop bandwidth of theamplifier 686. Specifically, by increasing the size of the capacitor C4,the bandwidth is decreased. A second amplifier 690 is used to providethe gain to the input terminal of the attenuator 318. Variable resistor692 is used to control the gain of the second amplifier 690.

One of the advantages of the present invention is that its optical traincan be implemented in a small-robust unit, possibly being integrated ona single optical bench. The following illustrates a number of differentembodiments showing various levels of integration.

FIG. 8 is a perspective view of one embodiment of the tunable laserlinear cavity configuration 200-1 illustrated in block diagram form inFIG. 2A. Specifically, the tunable laser 200-1 is integrated on a commonbench 800. Specifically, the SOA chip 610 is installed on a submount822, which holds the chip 610 off of the bench 800. The back facet 618of the chip 610 functions as the back reflector 618 of this linearcavity laser.

The chip's waveguide 611 generates light that is collimated by a firstlens component 820. Specifically, this lens component comprises a lenssubstrate S that is installed on the bench 800 via a mounting structureM.

The collimated light from the first lens component 820 passes through afirst isolator 614 or quarter waveplate to a second lens component 810.This second lens unit 810 focuses the light to launch it into the MEMSFabry-Perot tunable filter 612. Light in the passband, exiting from thetunable filter 612, is recollimated by a third lens component 812 topass through the second isolator 616 to a focusing lens component 822.

An endface 816 of the fiber 818 collects the light. A reflector 626 isprovided on the fiber to function as the output end of the laser cavity.This fiber 818 is then fiber coupled to the attenuator 318, if used. Afiber tap 320 directs a portion of the tunable optical signal from theattenuator 318 to the power detector 322.

Note that in other embodiments, the attenuator 318, tap 320, anddetector 322 are integrated in common on the bench 800 with the othercomponents of the tunable laser.

FIG. 9 is a perspective view of one embodiment of the tunable laserlinear cavity configuration 200-2 illustrated in block diagram form inFIG. 2B. Specifically, the tunable laser 200-2 is integrated on a commonbench 800. Specifically, the SOA chip 610 is installed on a submount822, which holds the chip 610 off of the bench 800. The back facet 618of the chip 610 functions as the back reflector 618 of this linearcavity laser.

The chip's waveguide 611 generates light that is collimated by a firstlens component 820. Specifically, this lens component comprises a lenssubstrate S that is installed on the bench 800 via a mounting structureM.

The collimated light from the first lens component 820 is launched intothe MEMS Fabry-Perot tunable filter 612. Light in the filter passband,exiting from the tunable filter 612, is recollimated by a third lenscomponent 822 to enter endface 816 of the fiber 818. The filter opticalaxis is tilted such that reflected light outside the filter passband isnot or minimally coupled back to the semiconductor chip waveguide. Areflector 626 is provided on the fiber to function as the output end ofthe laser cavity. This fiber 818 is then fiber coupled to the attenuator318, if used. A fiber tap 320 directs a portion of the tunable opticalsignal from the attenuator 318 to the power detector 322.

In more detail, a single spatial mode beam is emitted from the frontfacet of the SOA chip 610. This beam is typically collimated by lenscomponent 820 to form a diffraction limited waist.

The beam as launched into the tunable filter 612 is typically less than200 micrometers in diameter. Presently, it is less than 100 micrometersin diameter, being about 40 to 60 micrometers in diameter. In oneexample, the beam was 30-50 micrometers in diameter in a directionorthogonal to the top surface of the bench (y-axis direction) and 50-70micrometers in diameter in a direction that is parallel to the topsurface of the bench (x-axis direction).

With this configuration and angle a being about 1-2 degrees, there canbe some feedback reflection into the SOA chip 610. The small angle,however, improves the performance of the filter 612 by minimizing beamwalkoff. Currently α being about 1.1-1.5 degrees is considered the bestrange to maximize performance in view of these trade-offs minimizingfeedback into the SOA while maximizing the performance of the filter.

Note that in other embodiments, the attenuator 318, tap 320, anddetector 322 are integrated in common on the bench 800 with the othercomponents of the tunable laser.

FIG. 9A illustrates one embodiment of the optical train between thefront facet of the SOA 610 and the tunable filter 610. Specifically, thebeam displacement 6x enables the decoupling of the back reflection fromthe filter 610 and the SOA 610. Here is it is less than about 50micrometers, and preferably less than 20 micrometer, specifically about10 micrometers. In the example, the distance between the chip frontfacet ff and the filter 612 is less than about 10 millimeters. The focallength of the first lens component is less than 0.5 millimeters,specifically about 200 micrometers, or specified at 220 micrometers.

FIG. 10 is a block diagram showing one configuration of the ring laserillustrated in FIG. 2D. Here again, the components are installed on anoptical bench 800. Light is generated in the SOA chip 612 and collimatedby a first lens 820 to pass through an isolator 616 to a second lens 810that launches the light into the MEMS Fabry-Perot tunable filter 612.The light exiting from the tunable filter 612 is then recollimated by athird lens 812. A portion of the light exits from the tunable lasercavity as the tunable signal 210. The remaining light, however, isreturned to recirculate through the cavity.

In one implementation, this is achieved by a reflector or prism 632 thatis attached onto the bench 800. In other implementations, the light isrecirculated through the cavity using a length of polarizationmaintaining (pm) single mode fiber 635. The second embodiment hasadvantages in that the length of the optical cavity can be controlled.Specifically, the cavity can be made longer to increase the longitudinalmodal density.

In any event, the light returns along path 638, which can be above thebench 800 or provided by pm fiber length. Returning light is returned bya prism 634 through a second isolator 614 and a lens 632 to bereamplified in the SOA 612.

In one embodiment, the prism 634 is mounted to the bench 800 using apiezoelectric actuator unit 636. This allows for the location of theprism 634 to be moved in the direction of the X-axis, therebycontrolling the length of the optical cavity and therefore the spectrallocation of the longitudinal modes of the ring laser 100-4.

FIG. 11 is a perspective view of the ring laser source 200-4 illustratedin FIG. 2D. The components are installed on optical bench 800. Light isgenerated in the SOA chip 612 on submount 822. Its light is collimatedby a first lens 810 to pass through an isolating device, such as aquarter wave plate 616, to a second lens 820 that launches the lightinto the MEMS Fabry-Perot tunable filter 612. The light exiting from thetunable filter 612 is then recollimated by a third lens 812. A portionof the light exits from the tunable laser cavity as the tunable signal210 using tap or splitter 910 and fold mirror 912 and focusing lens 914.The remaining light, however, is returned to recirculate through thecavity via lens 916 and polarization-controlling or pm fiber length 638.

The light returns to lens 918, second isolator 614 and a lens 632 to bereamplified in the SOA 612.

FIG. 12 illustrates another embodiment of the tunable source 10, whichis based on source 200-3 of FIG. 2C.

This embodiment uses a tunable filter system 900, which includes anarray of tunable filters 612 and broadband light sources 622 in order toincrease the spectral width of the scanband. Typically, and in theillustrated embodiment, an array of five SLED chips 622 is mounted incommon on the bench 800. The light from each of these SLED chips 110 iscollimated by respective first lens components 918. Specifically, thereis a separate lens component 918 for each of these SLED chips 622.Separate isolators 920 are then provided for the broadband signals fromeach of the SLED chips 622.

An array of second lens components 922 is further provided to couple thebroadband signal into an array 900 of tunable filters 612. Specifically,separate Fabry-Perot tunable filters 612 are used to filter the signalfrom each of the respective SLED chips 622. Finally, an array of thirdlens components 926 is used to re-collimate the beam from the tunableFabry-Perot filters 612 of the tunable filter system 900.

For channel 1, C-1, a fold mirror 956 is used to redirect the beam fromthe SLED chip 622. The WDM filter 960 is used to combine the broadbandsignal from the SLED chip 622 of channel C-2 with the signal fromchannel C-1. Specifically, the filter 960 is reflective to thewavelength range generated by the SLED chip 622 of channel C-2, buttransmissive to the wavelength range of light generated by the SLED chip622 of channel C-1.

In a similar vein, WDM filter 962 is reflective to the signal bandgenerated by the SLED chip 622 of channel C-3, but transmissive to thebands generated by SLED chips 622 of channels C-1 and C-2. WDM filter964 is reflective to the light generated by SLED chip 622 of channelC-4, but transmissive to the bands generated by the SLED chips 622 ofchannels C-1, C-2, and C-3. Finally, WDM filter 958 is reflective to allof the SLED chips, but the SLED chip 622 of channel C-5. As a result,the light from the array of SLED chips is combined into a single tunablesignal 210.

A first tap 310 is provided to reflect a portion of the light throughthe etalon 312 to be detected by the wavelength detector 314. Then,another portion is reflected by tap 318 to the power detector 322. Theremaining tunable signal 210 is coupled by the fourth lens component 906into the optical fiber 818 via the endface 816.

The FIG. 12 embodiment can operate according to a number of differentmodes via a controller 410. Specifically, in one example, only one ofthe SLED chips in channels C-1 to C-5 is operating at any given momentin time. As a result, the tunable signal 210 has only a single spectralpeak. The full scan band is achieved by sequentially energizing the SLEDchip of each channel C-1 to C-5. This tunable signal is scanned over theentire scan band covered by the SLED chips of channels C-1 to C-5turning on the SLED chips in series, or sequentially.

In another mode, each of the SLED chips is operated simultaneously. As aresult, the tunable signal has spectral peaks in each of the scan bands,covered by each of the SLED chips 622 simultaneously. This systemresults in a more complex detector system, which must demultiplex theseparate scan bands from each of the SLED chips 622 from each of thechannels at the detector. Specifically, in one embodiment, five (5)detectors are used with a front-end wavelength demultiplexor.

FIG. 12 further shows a single bench fully integrated system accordingto still another embodiment. The sample detector system 12 is integratedon the same bench 800 and the tunable source. Specifically, lightreturning from the sample 10 in fiber 310 is coupled to sample detectorchip 334 using lens component 322.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

1. A hybrid cavity semiconductor laser comprising: an semiconductoroptical amplifier in a laser cavity; a tunable filter in the lasercavity; and a fiber forming part of the laser cavity, wherein the fiberis polarization controlling fiber.
 2. A laser as claimed in claim 1,wherein the fiber is polarization maintaining fiber.
 3. A laser asclaimed in claim 1, wherein the fiber propagates only a singlepolarization.
 4. A laser as claimed in claim 1, further comprising atleast one isolator for removing light reflected by the tunable filterfrom the cavity.
 5. A laser as claimed in claim 1, further comprising afilter lens component for coupling light between the amplifier and thetunable filter.
 6. A laser as claimed in claim 1, further comprising afiber lens component for coupling light between the filter and thefiber.
 7. A tunable ring laser comprising: an semiconductor opticalamplifier in a ring cavity; and a Fabry-Perot tunable filter in the ringcavity; and a length of polarization controlling fiber for returning thelight to recirculate through the ring cavity.
 8. A laser as claimed inclaim 7, wherein the fiber is polarization maintaining fiber.
 9. A laseras claimed in claim 7, wherein the fiber propagates only a singlepolarization.
 10. A tunable ring laser comprising: a semiconductoroptical amplifier in a ring cavity; a Fabry-Perot tunable filter in thering cavity; and prisms for returning the light to recirculate throughthe ring cavity.
 11. A tunable ring laser comprising: a semiconductoroptical amplifier in a ring cavity; and a Fabry-Perot tunable filter inthe ring cavity; and a cavity length modulator for controlling anoptical length of the optical cavity.
 12. A laser as claimed in claim 7,wherein the cavity length modulator controls a position of a reflectordefining part of the ring cavity.
 13. A laser system comprising: a lasercavity comprising a free-space portion and an optical fiber portion; asemiconductor gain medium in the free-space portion; a Fabry-Perottunable filter in the free space portion to control a wavelength ofoperation of the laser system to scan over a scan band tospectroscopically analyze a target.
 14. A laser system as claimed inclaim 13, wherein the Fabry-Perot tunable filter is amicroelectromechanical tunable filter.
 15. A laser system as claimed inclaim 13, wherein the Fabry-Perot tunable filter is a tunable thin filmfilter.
 16. A laser system as claimed in claim 13, wherein theFabry-Perot tunable filter is a thermally tuned thin film filter.